Device for receiving video signals and a method of controlling one such device

ABSTRACT

The invention relates to a device for receiving video signals comprising a first decoding chain, a second decoding chain, a video encoder and a selection means which is used selectively to connect the first decoding chain or the second decoding chain to the video encoder. The invention also relates to a method of controlling one such device.

The present invention relates to a device for receiving video signalsand to a method of controlling such a device.

To ensure their transport, whether through the air or by cable, videosignals are generally modulated by a carrier at a given frequency whichdepends on the program concerned. The video signal is thereafter broughtback to baseband by a tuner/demodulator assembly, also dubbed the frontend. Various processing operations may thereafter be performed on thesignals received by additional circuits as a function of the type ofsignal used.

When zapping from one program to another (hopping) at the request of theuser, the tuner aligns itself to a new carrier frequency therebybringing about the reception of a new signal and its possible processingby the additional circuits. In practice, these operations are notinstantaneous and a noticeable time span may elapse between the user'scommand and the displaying of the new program.

If no particular measure is taken, the display screen shows nothing(“black screen”) during this span of time, this of course beingprejudicial to the visual comfort of the user.

Solutions have already been proposed for attempting to remedy thisdrawback.

In patent applications EP 0 306 704 and JP 2000 032 338, it has beenproposed that during a change of program a still image emanating fromthe initial program be displayed until the new program can be displayed.In patent application EP 0 712 242, it has been proposed that apredetermined still image be displayed.

In patent application WO 99/16 247, images of type I (intra codingimages) contained in MPEG type streams corresponding to several programsare stored; during a change of program, if the new program is a programfor which an image of type I is stored, this (still) image is displayed.

These solutions only slightly improve visual comfort for the user:admittedly they replace the black screen by a still image, but they donot in any way improve the fluidity of display.

The invention proposes a novel solution for improving the visual comfortof the user upon a change of program: a device for receiving videosignals comprises a first decoding chain, a second decoding chain, avideo encoder and a means of selection for selectively connecting thefirst decoding chain or the second decoding chain to the video encoder.

According to other advantageous characteristics,

-   -   the device comprises a means of detection of the state of at        least one of the first and second decoding chains generating a        state cue and a means of control of the means of selection as a        function of the state cue;    -   the device comprises a user interface able to transmit a program        cue to the means of control;    -   the first and the second decoding chain are able to each        generate a digital video stream;    -   the first and the second decoding chain each comprise a video        decoder;    -   at least one of the first and second decoding chains comprises a        descrambler.

The invention also proposes a method of controlling such a device whichcomprises the following steps:

-   -   command of the means of selection for connection of the first        decoding chain to the video encoder;    -   reception of a signal comprising a program cue;    -   command of the second decoding chain for reception of the        program designated by the program cue;    -   maintenance of the command of the means of selection for        connection of the first decoding chain to the video encoder as        long as the second decoding chain is generating no video stream;    -   control of the means of selection for connection of the second        decoding chain to the video encoder when the second decoding        chain is generating a video stream.

The description of an exemplary embodiment of the invention will begiven with reference to FIG. 1 which represents the essential elementsof a digital decoder.

A digital decoder is a device for receiving video signals coded (ingeneral according to the MPEG standard), multiplexed and modulated by acarrier for their transport by cable or through the air (satellite or RFcommunication).

The digital decoder whose main elements are represented in FIG. 1comprises two decoding chains CHN1, CHN2.

Each decoding chain CHN1, CHN2 is connected at input to a common sourceSRC of video signals, for example an antenna or a cable of a localnetwork.

Each decoding chain CHN1, CHN2 comprises a tuner TUN1, TUN2, ademodulator DMD1, DMD2, a demultiplexer DMPX1, DMPX2 and an audio/videodecoder DEC1, DEC2 placed in series.

Each decoding chain CHN1, CHN2 thus generates, from the video signalsemitted by the common source SRC, a digital audio/video streamcorresponding to the program associated with the tuning frequency of thetuner TUN1, TUN2. The digital stream is for example of the 4:2:2 typeaccording to the CCIR 601 standard.

The output of each decoding chain CHN1, CHN2 is connected to an input ofa switch SW whose output is connected to the respective inputs of avideo encoder ENC and of an audio digital/analog converter DAC.

The switch SW makes it possible to selectively connect the output of thefirst decoding chain CHN1 or the output of the second decoding chainCHN2 to the audio converter DAC and to the video encoder ENC. The switchSW is controlled by a microprocessor CPU as explained later.

The video encoder ENC converts the digital stream that it receives fromthe first decoding chain CHN1 or from the second decoding chain CHN2into an analog video signal. The analog video signal is for example acomposite video signal (CVBS according to the usual initials) or asignal with three color components RGB. This analog video signal is madeavailable to the user on a connector VID (for example one or more pinsof a Scart socket) so as to be displayed on a conventional viewingdevice, for example a television set.

In a similar manner, the audio converter DAC converts the audio digitalstream into an analog audio signal heading for an output connector AUD,for example pins 1 and 3 of a Scart socket.

The microprocessor (here central processing unit) CPU is connected tothe various elements of the digital decoder for exchanging informationand commands with them. However, for the sake of clarity, only theconnections necessary for explaining the invention appear in FIG. 1.

In particular, for any connection between the microprocessor CPU and anelement of the second decoding chain CHN2, there also exists acorresponding connection (not represented) between the microprocessorCPU and the equivalent element in the first decoding chain CHN1.

The microprocessor CPU controls the switch SW through a control lineCTL. The microprocessor CPU also sends a frequency cue FREQ to the tunerTUN2 of the second decoding chain CHN2. This frequency cue FREQ is ingeneral an integer translated into frequency to be received by the tunerTUN2.

The microprocessor CPU also talks to a user interface UI from which itreceives in particular a program cue PRG. The user interface UIgenerally comprises an infrared receiver that receives infrared signalsfrom a remote control and translates them into a program cue PROG thatcan be understood by the microprocessor CPU. The program cue PROGindicates the program (that is to say the station or the channel) thatthe user wishes to receive.

The microprocessor CPU is also connected to the audio/video decoder DEC2of the second decoding chain CHN2 which sends it in particular a statecue RDY which indicates whether the audio/video decoder DEC2 is or isnot ready, that is to say whether it will be compressed without errorand emit a valid or invalid digital stream. This state cue RDY is forexample a software state of the driver (or pilot) that manages theaudio/video decoder DEC2.

A program change phase as proposed by the invention will now bedescribed.

For the sake of explanation it will be assumed that the switch SW linksthe first decoding chain CHN1 (more precisely its audio/video decoderDECL) to the audio converter DAC and to the video encoder ENC. Thedigital decoder therefore generates on its video VID and audio AUDoutputs analog signals of a program corresponding to the frequencyreceived by the tuner TUN1.

The program change phase (hopping) is initiated by the receipt of aprogram cue PRG by the microprocessor. In the example described here,the program cue PRG originates from a user interface UI (for example, atthe outset, from a remote control operated by the user). The inventionis however not limited to this example: the program cue PRG could alsooriginate from another video apparatus, such as a pre-programmed videorecorder that indicates to the digital decoder the program to bereceived by the latter so as to be recorded in the recorder.

The microprocessor CPU thereafter translates the program cue PRG into afrequency cue FREQ by means of a lookup table stored in a memory (notrepresented) associated with the microprocessor CPU.

The frequency cue FREQ is then sent to the tuner TUN2 of the seconddecoding chain CHN2. The tuner TUN2 will search for a signal at thefrequency indicated by the frequency cue FREQ, this not necessarilybeing immediate.

Once the tuner TUN2 has correctly received the desired signal (i.e. whenthe tuner TUN2 has locked on), the video signal received is transmittedthrough the other elements of the second decoding chain CHN2, namely thedemodulator DMD2, the demultiplexer DMPX2 and the audio/video decoderDEC2. Each element, and notably the audio/video decoder DEC2, generatesvalid signals at output only after a certain span of time relative tothe receipt of the signals at input.

According to a possible variant, a descrambler is introduced into eachdecoding chain between the demultiplexer and the audio/video decoder.The implementation of the descrambler also requires a certain span oftime since it is necessary to recover the keys that are encrypted in thestream and then to perform a digital computation to be able todescramble the audio and video streams.

It is important to note that, as long as the audio/video decoder DEC2 isgenerating no valid digital stream at output and thus the state cue RDYindicates that the audio/video decoder DEC2 is not ready, the command ofthe microprocessor CPU to the switch SW is maintained (such as it wasbefore the program change phase): the switch SW connects the firstdecoding chain CHN1 to the audio converter DAC and to the video encoderENC.

Throughout the start of the program change phase, as long as theaudio/video decoder DEC2 of the second chain CHN2 is not ready, thedigital decoder therefore continues to generate on its video VID andaudio AUD outputs analog signals corresponding to the program receivedby the first decoding chain CHN1.

When the audio/video decoder DEC2 of the second decoding chain CHN2generates a valid digital stream at output, the state cue RDY indicatesthat the audio/video decoder DEC2 is ready. When the microprocessor CPUreceives this state cue RDY “ready”, it modifies its command CTL to theswitch SW so that the latter connects the second decoding chain CHN2(and specifically the output of its audio/video decoder DEC2) to theaudio converter DAC and to the video encoder ENC.

From this moment onwards, the digital decoder therefore generates on itsaudio AUD and video VID outputs analog signals that lo correspond to theprogram received by the second decoding chain CHN2.

The change of program is therefore performed.

The next change of program may of course be performed in a similarmanner to the change of program described above by reversing the role ofthe first and of the second decoding chain.

The invention is of course not limited to the example described above.

Moreover, to simplify the figures, the connections between the variouselements are represented by a simple line even though in practiceseveral parallel connections (buses) are involved.

1-8. (canceled)
 9. A device for receiving video signals comprising: afirst decoding chain a second decoding chain; a video encoder; a meansof selection for selectively connecting the first decoding chain or thesecond decoding chain to the video encoder.
 10. Device as claimed inclaim 9, comprising: a means of detection of the state of at least oneof the first and second decoding chains generating a state cue; a meansof control of the means of selection as a function of the state cue. 11.Device as claimed in claim 10, comprising a user interface able totransmit a program cue to the means of control.
 12. Device according toclaim 9, in which the first and the second decoding chain are able toeach generate a digital video stream.
 13. Device according to claim 9,in which the first and the second decoding chain each comprise a videodecoder.
 14. Device according to claim 9, in which at least one of thefirst and second decoding chains comprises a descrambler.
 15. A methodof controlling a device as claimed in claim 9, comprising the followingsteps: command of the means of selection for connection of the firstdecoding chain to the video encoder; reception of a signal comprising aprogram cue; command of the second decoding chain for reception of theprogram designated by the program cue; maintenance of the command of themeans of selection for connection of the first decoding chain to thevideo encoder as long as the second decoding chain is generating novideo stream; command of the means of selection for connection of thesecond decoding chain to the video encoder when the second decodingchain is generating a video stream.
 16. Method of controlling a deviceas claimed in claim 10, comprising the following steps: command of themeans of selection for connection of the first decoding chain to thevideo encoder; reception of a signal comprising a program cue command ofthe second decoding chain for reception of the program designated by theprogram cue; maintenance of the command of the means of selection forconnection of the first decoding chain to the video encoder as long asthe state cue of the means of detection indicates that the seconddecoding chain is generating no video stream; control of the means ofselection for connection of the second decoding chain to the videoencoder when the state cue of the means of detection indicates that thesecond decoding chain is generating a video stream.
 17. Method ofcontrolling a device as claimed in claim 11, comprising the followingsteps: command of the means of selection for connection of the firstdecoding chain to the video encoder reception of the program cue fromthe user interface; command of the second decoding chain for receptionof the program designated by the program cue; maintenance of the commandof the means of selection for connection of the first decoding chain tothe video encoder as long as the state cue of the means of detectionindicates that the second decoding chain is generating no video stream;control of the means of selection for connection of the second decodingchain to the video encoder when the state cue of the means of detectionindicates that the second decoding chain is generating a video stream.